Preheat oven to 180 degrees C. Mix together crumbs, 1/4 cup sugar and 65g melted butter or margarine. Press mixture into the bottom of a springform pan. Bake 10 minutes. Cool. Don't turn the oven off.
Place potatoes in a baking dish. Bake until a knife inserted in centre goes through easily, about 1 hour. Don't turn oven off. Cool sweet potatoes enough to handle, peel and puree.
Transfer 1 1/2 cups of sweet potato puree to a large bowl. Add cream cheese, 7/8 cup white sugar, sour cream and 1/4 cup cream; beat until smooth. Beat in eggs one at a time, blending well after each. Pour filling into crust.
Bake until tester inserted in centre comes out clean, 1 hour. Turn off the oven. Let cake stand 1 hour in oven with door ajar.
Combine brown sugar and 65g butter or margarine in a heavy small saucepan. Stir over low heat until sugar dissolves. Increase heat, and bring to a boil. Mix in 1/4 cup cream then nuts. Pour hot topping over cheesecake. Refrigerate.